Output Phase and Amplitude Analysis of GaN-Based HEMT at Cryogenic Temperatures

We report both output phase and amplitude performance for gallium nitride (GaN)-based high-electron-mobility transistor (HEMT), operating in RF small- and large-signal modes from 270 K to cryogenic 70 K. Intrinsic elements of the device were extracted employing a small-signal equivalent circuit model. Temperature dependences of maximum available power gain, short-circuit current gain, and open-circuit voltage gain indicated performance improvement as lowering the ambient temperature. Meanwhile, the output phase of small RF signal measurement was found to be decreased linearly with decreasing the temperature. In RF large-signal operations, power and phase nonlinearity induced by AM-to-AM and AM-to-PM conversion were characterized via gain and phase compression, respectively. Measured
$P_{\mathrm {1\,dB}}/P_{\mathrm {3\,dB}}/P_{\mathrm {5\,dB}}$
was decreased monotonously for
$T > 150$
K, whereas it showed little temperature sensitivity under 150 K. Output phase shift was expanded as increasing the input power level and was all greatly suppressed as dropping the temperature.